This invention relates to methods and apparatus for electrostatically charging a surface and more particularly to improved corona generating methods and apparatus therefore usable in electrostatic recording and reproducing equipment or in any other application where it is desirable to efficiently charge a selected medium.
In electrostatic recording and reproducing processes such as the electrophotographic process known as xerography, it is necessary to sensitize a photoreceptor structure by charging at least one surface thereof to a potential which is preferably uniform. Subsequent to or simultaneously with the sensitizing of the photoreceptive strucutre in such electrophotographic processes, the photoreceptor structure is exposed so that a photosensitive layer therein is rendered selectively conductive whereupon a latent electrostatic image is formed which may be then developed using conventional electrophotographic techniques. The developed image is then transferred onto a copy substrate on which it is rendered permanent by means of a fixing process.
In the above-noted electrophotographic process, electrostatic charging techniques are generally relied upon to accomplish such necessary processing steps as the transfer of an electrostatically formed image from a reusable photoreceptor structure to a transfer member and/or tacking and stripping operations associated with such transfer member. While many forms of acceptable techniques for electrostatically charging a surface are known, corona discharge techniques have generally been preferred in applications such as those mentioned above because such techniques are particularly well suited to applying an electrostatic charge to a moving surface and the use of corona discharge techniques allows a selected surface to be rapidly charged to a relatively high potential. Furthermore, since corona generating apparatus generally employ a wire-like electrode, they are advantageous because the charging process involved acts to impose a potential level on the surface being charged which tends to be more uniform than that obtained from other surface charging techniques. Conventional forms of corona generating apparatus are illustrated in U.S. Pat. Nos. 2,836,725 and 2,879,395 and generally comprise one or more wire-like electrodes, known at coronodes, horizontally disposed above the surface to be charged and a shield which may take a plurality of different structural forms, partially disposed about the coronode. In one conventional mode of operation, a high voltage D.C. power supply is connected to the coronode with the requisite polarity for the charging operation which is desired, while a conductive layer associated with the surface to be charged is grounded as are the other terminals of the power supply and the shield. Conventional corona generating apparatus, when used according to the foregoing mode of operation are notoriously inefficient devices in that only a small percentage of the corona current produced at the coronode is delivered to the surface to be charged while a relatively large percentage of the corona current is diverted to the shield and is thereby wasted from the standpoint of applying an electrostatic charge to the surface to be charged.
In an alternate mode of conventional corona generating apparatus operation, a D.C. power supply is connected to the coronode and a conductive layer associated with the surface to be charged while the shield is left unconnected or electrically open. This alternative mode of operation is highly efficient from the standpoint of the current delivered because no current may be diverted by the shield. However, it is much less efficient from the power utilization standpoint because the voltage which is required to be applied to the coronode is much larger than in previously mentioned corona generating apparatus configurations and hence the power supply relied upon must be physically large and capable of producing very large potential levels.
Another arrangement for biasing a corona discharge device in a xerographic machine is taught in U.S. Pat. No. 2,686,989, wherein the corona wire is biased to a high positive corona generating potential, the conductive substrate on which the photoconductive material is held is maintained at ground potential, and the shield is biased to a potential intermediate the corona wire potential and ground. This method also suffers from the disadvantage of a high current to the shield.
The conclusion to be drawn from prior experience with the above different biasing arrangements is that while grounding the shield (or biasing it to various intermediate potentials) promotes corona current at relatively low coronode potential, maintenance of such constant potentials at the shield reduces the efficiency of the corona device by attracting a large percentage of current to the shield.